As modern commerce depends on reliable and cost-effective methods for delivering products from suppliers to users, the availability of durable and reliable highways, roads, and other support surfaces for vehicles is vital for sustaining a modern economy. To provide better support surfaces, highways, roads, and sidewalks are commonly paved with a layer or mat of asphaltic concrete that is laid over the surface of the sub-base. Asphalt is preferred over cement to pour roads because it is less expensive and very durable. Asphalt can also be poured at night, which allows major roads to be shut down at the least busy of times for maintenance. Relative to road noise, asphalt is also quieter than cement, making it the better choice for roads.
Asphalts are essentially mixtures of bitumen, as binder, with aggregate, in particular filler, sand, and stones. There are many different types of asphalts available and their characteristics can vary quite significantly. The design of asphalts for bituminous paving applications is a complex process of selecting and proportioning materials to obtain the desired properties in the finished construction while minimizing undesirable characteristics.
In evaluating and adjusting mix designs, the aggregate gradation and the binder content in the final mix design are balanced between the stability and durability requirements for the intended use. The final goal of a mix design is to achieve a balance among all of the desired properties. Many binders and various polymers have been investigated for reaching similar goals, and other modifications have been studied.
Unsaturated thermoplastic elastomers like styrene-butadiene-styrene (SBS) block copolymers are polymers used for asphalt modification. These polymers enhance the elastic recovery capacities of asphalt and, therefore, its resistance to permanent deformations. However, unsaturated elastomeric polymers are quite expensive and are subject to degradation when exposed to atmospheric agents and mechanical stress. Due to their fragility, they are typically used as virgin polymers, which can result in significant cost increases for the product. While SBS is recognized for its performance benefits, research has focused on more cost effective modifiers in exchange for sacrificing superior performance.
Olefinic polymers have been investigated for use as modifiers. They are available in large quantities with different mechanical properties and at low cost. Polyethylene (PE) and polypropylene (PP) are plastomers. They impart high rigidity (i.e., lack of elasticity, resistance to bending) to the product and significantly reduce deformations under traffic load. Due to their non-polar nature, PE and PP are almost completely immiscible with asphalt, and are thus limited in use.
Conventional asphalts often do not retain sufficient elasticity in commercial use and exhibit plasticity ranges which are too narrow for use in many modern applications, such as road construction. The characteristics of road asphalts can be improved by incorporating an elastomeric-type polymer. There exists a wide variety of polymers that can be mixed with asphalt, with SBS being a commonly used polymer in asphalt modification. The resulting modified asphalts are commonly referred to as bitumen/polymer binders or asphalt/polymer mixes. There is a need for hot mix asphalt concrete modifiers that would increase resistance to permanent deformation while maintaining or increasing the modulus of the mix at intermediate temperatures without significantly affecting the binder properties.
The bituminous binders, including the bitumen/polymer type, presently employed in road applications often do not have optimal characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration. The current practice is to add the desired level of a single polymer, sometimes along with a reactant which promotes cross-linking of the polymer molecules until the desired asphalt properties are met. The reactant is typically sulfur in a form suitable for reacting.
When added to bitumen at 140° C., sulfur is finely dispersed in bitumen as uniformly small particles, with coagulation and settlement of the sulfur particles becoming noticeable after a few hours. Therefore, the sulfur extended asphalt (SEA) mixtures can be produced directly in the mixing plant just before the laying of the asphalt mixture. One major concern in handling sulfur-asphalt mix is the fear of the evolution of hazardous hydrogen sulfide (H2S) during production and laying. This problem can be ameliorated by adding carbon or ash to sulfur. H2S evolution starts at temperatures higher than 150° C., so that the application of the asphalt mixture at temperatures up to 150° C. avoids pollution and safety problems. However, H2S evolution starts well below 150° C., i.e. about 130° C., which is undesirable from an environmental perspective. Moreover, below 120° C., neither the reaction of the asphalt and sulfur nor the cross-linking of the SBS/sulfur blend could take place.
Besides the performance and environmental issues associated with many types of asphalt modifiers, many of the polymers that are used to modify asphalt compositions are expensive and can be difficult to obtain in remote areas of the world.
A need therefore exists for a filler that can be used in asphalt compositions. Historically, limestone powder, limestone dust, and cement dust have been used as filler.